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#96625 - 12/13/05 08:54 PM GFP and EGC impedance requirements  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Does the use of ground fault detection change the performance requirements for equipment grounding conductors?

This came up in the question about voltage drop in the EGC in long feeders.

One of the things that confused me was the change between the 2002 and the 2005 code. In the 2002 code there was no clear requirement that the ground fault path actually be capable of tripping the breaker. But the requirement to facilitate the operation of the over-current device is now explicit. IMHO this was probably an unintentional omission from the 2002 code, seeing as it was a requirement for ungrounded systems in the event of a double fault.

Soares Chapter 11 provides references for the maximum impedance that can be expected to trip a breaker in the instantaneous trip range. This seems a reasonable criterion for 'facilitating operation'; in the event of a short circuit the breaker would trip at its fastest possible rate. However the code itself doesn't seem to specify what 'facilitating operation' means is useful quantitative terms; Soares suggests that tripping in 1 cycle is 'fast enough', but what about 10 cycles or 1 second?

My question: if the overcurrent device incorporates ground fault detection, does this permit increased impedance in the EGC circuit? With ground fault detection, operation of the breaker would happen at ground faults well below the normal trip rating of the breaker. I am not asking if this would permit smaller EGC conductors than other portions of article 250; but if ground fault detection would assist in meeting the performance requirements of 250.4(A)(5)?


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#96626 - 12/13/05 10:33 PM Re: GFP and EGC impedance requirements  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
Jon- Yor're a smarter man than I am for sure but if I understand your question, you are talking about an "ungrounded" system and the 250.4(A)(5) is under the heading of a "grounded" system. I detect (no pun intended) you are talking about an ground fault dectection on a "ungrounded" system. In either case I could find no provision for reducing the EGC for either a grounded or ungrounded system. If your talking about the conductors used for the ground fault sensor, then I would say they don't need to be sized per 250.122 like the EGC. Hope I'm on the right wave length with your question.

George Little

#96627 - 12/14/05 12:26 AM Re: GFP and EGC impedance requirements  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Sorry that I mentioned the ungrounded systems; that was just a distraction. In the 2002 code, 250.4(B) for ungrounded systems had a requirement that the EGC be sufficient to facilitate the operation of OCPD...but 250.4(A) did not have a similar requirement for grounded systems. This discrepancy was rectified in the 2005 code. 250.4(A)(5) includes a requirement for 'facilitating the operation of the overcurrent device...' (there is also a confusing use of 'high-impedance grounded systems', which might limit the applicability of 250.4(A)(5), but I really don't want to go there [Linked Image] )

As I read it, 250.4(A)(5) is an additional requirement, above and beyond the sizing required by 250.122. In 250.122 you are giving a minimum size requirement for the EGC. Additionally, the performance requirement of 250.4(A)(5) must be met, which might require a larger EGC in some situations.

I am trying to understand the performance requirements of 250.4(A)(5), and to understand if GFP changes these requirements. I guess I imagine a situation where with a normal breaker 250.4(A)(5) would require a larger EGC than 250.122, but where the use of GFP would permit using something sized to 250.122.


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